132 PRIMORDIAL—WAPPINGER LIMESTONES. 
fucoid-like tracery ; the weathered surfaces are often 
rough with sand-grains. This variety passes into one 
which differs chiefly in being fissile into more or less 
thin slabs; this variety often alternates with layers 
of exceedingly thin and friable shale, the folia of which 
are covered with loose sand as they decompose. 
The rock also passes on the one hand into argillaceous 
varieties represented by a smooth, fine-grained, massive 
argillaceous limestone well exhibited on the west side 
of the main fossiliferous hill ; and represented also by a 
very fissile and smooth calcareous shale, well shown in 
a hill (K) which is a continuation, in the second field 
south, of the above-mentioned hill. Although nearly as 
fissile, this variety differs essentially in appearance from 
the closely adjoining Hudson river shales, which are 
much darker, and more glazed or shining on their sur- 
faces. On the other hand, this Potsdam rock passes 
into a very solid, massive quartzyte, the fine sand-grains 
of which are everywhere in absolute contact, so that in 
appearance it is only a grade less compact than the 
quartzyte of Stissing mountain ; but the minute inter- 
stices are occupied by calcareous material, so that the 
rock effervesces a little under acids. 
In many places, again, the material takes the form of 
brecciated conglomerate ; this is well shown in the west- 
ern Potsdam hill at the northern extremity of the tract 
under consideration (hill B), and its extension south 
into the next field (hill D); also in the western margi- 
nal Potsdam ridge just south of T. A. Hinkle’s house in 
the next field. 
The southeastern extremity or fork of the main fos- 
siliferous hill (A) is a solid mass of peculiar odlyte, 
made up of spherules which are simply aggregations of 
rhomboidal calcite crystals imbedded in the interstitial 
mass of quartzyte or calcareous quartzyte. I have 
found pebbles of this peculiar odlyte in the conglomerate 
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